Hong Kong's biggest sewage-treatment plant is capable of recycling about 1,000 cubic metres of water a day, thanks to an upgraded purification system.

The 28-hectare Sha Tin Sewage Treatment Works next to the racecourse handles waste water from Sha Tin and Ma On Shan districts, whose residents produce 250,000 cubic metres of sewage a day.

The sewage used to undergo basic treatment and be sent to Tolo Harbour, but since November an improved system has greatly enhanced cleanliness of the treated water, increasing its usage.

The new system purifies water with three additional delicate parts: disc filters, ultra-filtration membranes and reverse osmosis membranes. The cost of the entire water-reclamation facility is about HK$7.2 million.

Disc filters catch particulates exceeding 130 micrometres, and ultra-filtration 0.03 micrometre as well as most bacteria. Finally, the filtrate flows through reverse osmosis membranes where viruses, salts and other substances exceeding 1 nanometre in diameter are barred.

Reverse osmosis is a filtration method that removes many types of large molecules from solutions by applying pressure to the fluid when it is on one side of a membrane. That is considered an advanced technology, and this is the first time the Drainage Services Department has adopted it.

To have an idea of how small 1 micrometre is, a human hair's diameter is about 90 micrometres.

The reclaimed water produced each day - about 1,000 cubic metres - is enough to cater to about 70 per cent of the water treatment plant's needs for such things as toilet flushing and diluting chemicals.

There were plans to expand the use of the water even further, for instance to New Territories farms so farmers could save more drinking water, said Eddie Pak Kan-ming, a senior engineer at the Drainage Services Department.

However, the water was unsuitable for drinking because it lacked certain chemicals and may not be clean enough, he said. But water treated in this plant was not designed for drinking, but to help alleviate the global problem of freshwater shortage, Pak said.

About 70 to 80 per cent of Hong Kong's fresh water is imported from the Dongjiang, or East River, over a distance of more than 80 kilometres. The agreed maximum quantity of Dongjiang water for Hong Kong is 1.1 billion cubic metres a year.

But that will not be enough after 2030, according to a government forecast. So reclaiming water was one of the solutions the government devised in 2007, alongside reducing water consumption, protecting water resources and exploring new ones.

Cleaning up

The sewage treatment plant uses state-of-the-art technology

The total cost, in HK dollars, of the upgraded water-reclamation facility is:$7.2m